Certain metals, such as silver, have been known to be effective antimicrobial agents. Generally, an antimicrobial metal can be combined with certain polymer, such as polyester, to form an antimicrobial complex. This antimicrobial complex can then be mixed into another polymer, i.e., a base polymer, to form a combined antimicrobial polymer composition. Finally, such antimicrobial polymer composition can be incorporated into coating, paint, and plastics products for hygiene and medical device applications.
Conventional antimicrobial polymer compositions have one shortcoming, however. While it is well known that a polymer composition containing the aforementioned metal-based antimicrobial complex provide excellent anti-bacterial properties, when products containing the antimicrobial polymer composition comes in contact with heat or light, it often change color.
The point in time when discoloration of an antimicrobial polymer composition containing a metal-based antimicrobial complex appears can range from early in the manufacturing process, to late in a finished article's useful life. For example, discoloration of the antimicrobial polymer can occur right after mixing the metal-based antimicrobial polymeric additive with the base polymer composition. Likewise, discoloration can occur due to photo-instability resulting from short or long term exposure to high-energy radiation such as ultraviolet (UV) radiation from sunlight.
There had been a number of efforts to overcome this discoloration problem. These efforts, however, exhibit various limitations or are overly complex.
For example, WO2006051028 discloses a process of preventing discoloration of antimicrobial polymer compositions during exposure to sun light by adding a UV-absorber or a mixture of UV-absorbers. In particular, the disclosed process describes first forming a base polymer composition by mixing a monomer, such as an acrylic or methacrylic acid derivative, with a lauroylperoxide initiator. Thereafter, a common UV-absorber or a mixture of UV-absorbers, such as hydroxybenzophenone, 2-hydroxyphenylbenzotriazole, 2-hydroxypheyltriazine, and oxanilide, is added to the base polymer before adding the antimicrobial polymeric complex.
In another example, U.S. Patent Publication No. US2008/0242794 discloses adding a color stabilizer, such as a bromated or iodate ion, to a thermal-initiated polymer composition containing a silver-based antimicrobial polymeric complex. The disclosure also describes a process of preparing such a polymer composition by either mixing the color stabilizer with the base polymer composition before adding the silver-based antimicrobial polymeric complex, or compounding the color stabilizer with the base polymer simultaneously with the addition of the silver-based antimicrobial agent.
Some even proposed reducing discoloration by simply combining a silver-based antimicrobial agent with other metals, in hopes of lowering the total amount of silver in a given formulation. For example, U.S. Patent Application No. US20060156948A1 describes mixing copper with silver in the antimicrobial polymeric complex to lower the level of silver ion, knowing that copper is a less reactive but also a less potent antimicrobial metal.
In sum, the existing efforts appeared in the art for solving the discoloration problem in antimicrobial polymers, such as the ones mentioned above, have been limited to either reducing the amount of potent antimicrobial agents or adding remedial substances such as color stabilizers and UV-absorbers to the base polymer. The existing efforts require either sacrificing antimicrobial efficacy (such as mixing copper with silver), or incurring additional costs and processing time (such as adding UV-stabilizers and absorbers). Therefore, the existing solutions focus generally on minimizing or remedying the discoloring symptoms.
Hence, there remains a need to come up with a solution to eliminate the fundamental cause of the discoloration problem that exists in polymer compositions containing metal-based antimicrobial agents.